Talk:Torque

old units

Sam said that torque is lbf·ft, not ft·lbf The sequence force·distance is used almost without exception in metric units, so that is a parallel. I am aware not aware of any reason for the sequence. The BIPM seems to have a consistent sequence for units but I am not sure what the guideline is. Do we have any statistics on how often the foot unit is first or second? Google comes up with many for either sequence. Perhaps it doesn't really matter. I don't mind either way. Bobblewik  (talk) 17:22, 27 Apr 2005 (UTC)

The sequence distance-force was common in the old metric "meter-kilograms" and "centimeter-kilograms", more common than "kilogram-meters", the order common for energy. (I have a torque wrench in "meter kilograms", for example.) Google can find several of them as well.

Both "pound-feet" (lbf·ft) and "foot-pounds" (ft·lbf) are in common use, as you discovered. Some experts and style guides recommend the former to distinguish the units of torque from the units of energy/work, a different quantity. See this, for example. But there is much less of a sense of generally accepted rules for the English units (or for the old metric units, for that matter) than there is with SI. Gene Nygaard 17:44, 27 Apr 2005 (UTC)

I've written this up under units. Thanks for the good questions! Samw 21:59, 27 Apr 2005 (UTC)

Your link was bogus. Neither it nor anything else on the BIPM site says anything about the units of torque.

Futhermore, there are sources recommending pound-feet rather than foot-pounds going back to before SI ever existed. So your chronology was wrong there, too. Gene Nygaard 22:30, 27 Apr 2005 (UTC)

Sorry, I didn't notice the second table on that page. Of course, they call it moment of force there, ratrher than torque—do you take that as prescriptive as well?

Here's what they say:

"A derived unit can often be expressed in different ways by combining the names of base units with special names for derived units. This, however, is an algebraic freedom to be governed by common-sense physical considerations. Joule, for example, may formally be written newton metre, or even kilogram metre squared per second squared, but in a given situation some forms may be more helpful than others.

"In practice, with certain quantities preference is given to the use of certain special unit names, or combinations of unit names, in order to facilitate the distinction between different quantities having the same dimension. For example, the SI unit of frequency is designated the hertz, rather than the reciprocal second, and the SI unit of angular velocity is designated the radian per second rather than the reciprocal second (in this case retaining the word radian emphasizes that angular velocity is equal to 2pi times the rotational frequency). Similarly the SI unit of moment of force is designated the newton metre rather than the joule."

So we can probably add something along those lines in the first paragraph of that subsection.

Now let's deal with the silly notion that the order of terms in English units should be determined by the order of terms in SI units. That's nonsense.

As I pointed out above, and as you can determine for yourself by searching the internet with your favorite search engine, when kilograms-force are used, the "meter-kilogams" order is more common than the "kilogram-meters" order (perhaps in part due to he tendency of people to be sloppy in the separation of their units of measure, as can be seen on all the Wikipedia pages still using "Nm" as a symbol for newton-meters, and due to the fact that before SI, "gm" was an acceptable symbol for grams, there would be ambiguity that would need to be resolved if kilogram-meters were used. Is "kgm" the symbol for kilograms, or for kilograms multiplied by meters&that's the question people would have to ask.

As I claimed above, the idea that we should distinguish the English units of torque from the English units of energy and work far predates the International System of Units, which was only introduced in 1960. I have found a clear example:

A.M. Worthington, Dynamics of Rotation: An Elementary Introduction of Rigid Dynamics, London, New York, Bombay, Calcutta, and Madras: Longmans, Green, and Co., 1920., p. 9.

"British Absolute Unit of Torque.–Since in the British absolute system, in which the lb. is chosen as the unit of mass, the foot as unit of length, and the second as unit of time, the unit of force is the poundal, it is reasonable and is agreed that the British absolute unit of torque shall be that of a poundal acting at a distance of 1 foot, or (what is the same thing, as regards turning) a couple of which the force is one poundal and the arm one foot. This we shall call a poundal-foot, thereby distinguishing it from the foot-poundal, which is the British absolute unit of work.

"Gravitation or Engineer’s British Unit of Torque.–In the Gravitation or Engineer’s system in this country, which starts with the foot and second as units of length and time, and the pound pull (i.e. the earth’s pull on the standard lb.) as unit of force, the unit of torque is that of a couple of which each force is 1 pound and the arm 1 foot. This may be called the ‘pound-foot.’"

Nonetheless, there is no clear standard for English units today.

BTW, Worthington, who named the "slug" as a unit of mass in an earlier (1904) edition of this book (which may well have contained much of the same information about torque), also had another bright idea:

"In the interests of clear teaching, the convention (which I am glad to see has been adopted in America) has been adhered to throughout, of using the word ‘pound’ when a force is meant, and ‘lb.’ when a mass is meant, and I have ventured to give the name of a ‘slug’ to the British Engineer’s Unit of Mass, i.e. to the mass in which an acceleration of one foot-per-sec.-per-sec. is produced by a force of one pound.'

It's easy enough to see why that idea didn't catch on. The people who use the mass units would like to have both a spelled out word and a symbol for them, as would the people who use the force units (and, of course, those are often the same people). Gene Nygaard 23:05, 27 Apr 2005 (UTC)

Thanks for the great examples. Of course it was silly of me to assume imperial conventions come from SI. However, I stand by my assertion that lbf-ft predominates over ft-lbf for whatever reason and I suspect that's why SI adopted N-m. See googlefight: http://www.googlefight.com/index.php?lang=en_GB&word1=%22lbf-ft%22+torque&word2=%22ft-lbf%22+torque How can we writeup this factoid? Samw 00:01, 28 Apr 2005 (UTC)

I think that if you looked into it carefully, you would find significant differences in usage in different fields of activities, geography, etc. More a matter of degree than of uniform usage in any particular domain.

It is my impression that when it comes to automobile engine torque, the foot first versions predominate, pretty much everywhere (even as ft-libras and the like). But you will find variations among different manufacturers, variations among different magazine editors, etc., and in large groups little consistency at all. On Wikipedia, ft·lbf clearly predominates over lbf·ft in this context, but that is probably still true but not so overwhelming outside Wikipedia.

I think the feet first is even more predominant when it comes to torque wrenches, and specificantions in shop manuals and installation instrucions and the like, for how much torque to apply to bolts. Gene Nygaard 01:25, 28 Apr 2005 (UTC)

A question about redirects

I noticed the following question at User talk:Sfoskett.

• When I copied the information from the LT4 page, I assumed "ft.lbf" was a typo, but I see now that it's actually a pretty common way of abbreviating "foot-pounds". It would be somewhat difficult for someone who knew little of US units to figure this out, since no search on Wikipedia ("ft·lbf", "ft.lbf", "ft lbf", and so on) will actually get you to the foot-pound page. Do you think it would make sense to put a redirect in from ft·lbf to Foot-pound and possibly link it (sparsely or at least initially) in articles? I'm somewhat new here and not sure that it's a valid use for redirects, but I think it would be useful. —HorsePunchKid→龜 23:13, 2005 Jun 16 (UTC)

I thought that people here might have something to say about it. Bobblewik  (talk) 12:42, 20 Jun 2005 (UTC)

I added Wikilinks for ft·lbf and ft.lbf going to Foot-pound. We ought to linkify the first instance of this unit anyway, since it's unfamiliar to many. --SFoskett 13:00, Jun 20, 2005 (UTC)

Sin/Cos

Can someone please verify that this is the correct formula for the magnitude of torque: ${\boldsymbol {T}}=({\textrm {distance\ to\ centre}})\times \cos(\theta )\times {\textrm {force}}$ An anon keeps changing it to sin instead of cos... --SFoskett 17:05, July 18, 2005 (UTC)

Force applied at an angle; projection onto perpendicular is given by the cosine.

I had been wondering about that, but I hadn't really cared enough to work it out until you mentioned it, mainly because I figured it would depend on exactly how the problem is phrased. The cosine version is correct as the problem is stated. Give me a few minutes here, and I'll have a better diagram. —HorsePunchKid→龜 19:46, July 18, 2005 (UTC)

Well, there's a diagram. Not my best work, but if you think it might help clarify the formula, we can find a place in the article for it. —HorsePunchKid→龜 20:08, July 18, 2005 (UTC)

Sine/Cosine

Here's a little blurb to lay the sine/cosine issue to rest:

First of all, the cosine term in the definition of torque listed on this page is NOT correct. Torque is a result of a vector cross product, that is:

T = r x F

The definition of a cross product in scalar form is the following:

T = (r)(F)[sin(theta)]

Theta is the angle between 'r' and 'F'; while the diagram on the page is correct in stating that the side in question is (F)[cos(theta)], that is not the correct quantity to use when determining torque. The REAL definition is above, and a proof is located in Analytical Mechanics: Sixth Edition by Grant Fowles and George Cassiday, on pages 15-16. In order to use the angle theta as defined in the picture, the correct equation would have to be:

T = (r) (F) {[1-((cos^2)(90-theta))]^(1/2)}

This is true for all cross products. The angle in question is ALWAYS the angle between the two vectors. The presence of a cosine term in the multiplication of two vectors implies a dot product, rather than a cross product.

FYI, the 'Anon' making the edits has a BA in Physics from the University of Maine, and I'm not too far behind; I'm currently finishing up my senior year in Physics at UMaine.

SirMink, 25 July 2005, 09:00 EST.

Cross product vs. scalar multiplication

I find it very confusing that the same symbol $\times$ is used for both cross product and multiplication of two scalars, as in $a\times b$. I've never seen this symbol used for multiplication (except maybe in elementary school). Usually, the product of two scalars $a\$ and $b\$ is written as $a\cdot b$ or simply $a\ b$.

The problem could be that the symbol $\times$ is called \times in TeX, and someone took this too literally?

You get confused too easily. Why is it okay to use the dot for scalar multiplication but not the cross? Wouldn't you instead get confused with the dot product?

Proposed Phrase

"Do as I torque, not as I talk!"

"Do as I do, not as I say!".

Hilarious

"Loosely speaking, torque is a measure of the turning force on an object such as a bolt" ... I love that quote. Torqur turns Loose bolts! Hahahahahha!

Inline math notation

Moved to WT:MSM § Inline math notation